Import each frame into photoshop stack (File, Scripts, Load Files into Stack)Select all layersSelect blending mode to LightenAdd a threshold adjustment layer and adjust as desiredAdd a curves adjustment layer and adjust as desired ( I used the double arrowed icon on lhs and dragged down on a dark area)

notes

There's quite a lot of light pollution where the shots were taken. the bright trails show the stars of Ursa Major (big dipper)